Analog interferometric modulator

ABSTRACT

Methods and devices for calibrating and controlling the actuation of an analog interferometric modulator configured to have a plurality of actuation states. Devices and methods for calibrating an analog interferometric modulator to respond in linear relation to an applied voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims priority to and is a continuation of U.S. patentapplication Ser. No. 12/485,003, filed Jun. 15, 2009, entitled “ANALOGINTERFEROMETRIC MODULATOR,” and assigned to the assignee hereof. Thedisclosure of the prior application is considered part of, and isincorporated by reference in, this disclosure.

BACKGROUND

1. Field of the Invention

The present invention relates to driving schemes and calibration methodsfor analog interferometric modulators.

2. Description of Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that absorbs and/or reflects light in a spectrally selectivemanner using the principles of optical interference. In certainembodiments, an interferometric modulator may comprise a pair ofelectrically conductive plates, one or both of which may be transparentand/or reflective in whole or part and capable of relative motion uponapplication of an appropriate electrical signal. In a particularembodiment, one plate may comprise a stationary layer deposited on asubstrate and the other plate may comprise a metallic membrane separatedfrom the stationary layer by an air gap. As described herein in moredetail, the position of one plate in relation to another can change theoptical interference of light incident on the interferometric modulator.Such devices have a wide range of applications, and it would bebeneficial in the art to utilize and/or modify the characteristics ofthese types of devices so that their features can be exploited inimproving existing products and creating new products that have not yetbeen developed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable minor position versus applied voltage forone exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a cross-section of an embodiment of an interferometricmodulator with three layers in which a movable middle layer is in arelaxed position.

FIG. 9 is a cross-section of an interferometric modulator of theembodiment of FIG. 8 with resistive elements disposed on the firstlayer.

FIG. 10 is a cross-section of an interferometric modulator of theembodiment of FIG. 8 with a control circuit.

FIG. 11 a cross-section of is an interferometric modulator of theembodiment of FIG. 8 illustrating different actuation positions.

FIG. 12 is an illustration of an interferometric modulator of theembodiment of FIG. 8 designed to compensate for parasitic capacitance.

FIG. 13 is a flowchart of an embodiment of a process of calibrating theinterferometric modulator of the embodiment of FIG. 9.

FIGS. 14A-14E illustrate an interferometric modulator of the embodimentof FIG. 9 in various stages of an embodiment of a calibration process.

DETAILED DESCRIPTION

The following detailed description is directed to certain specificembodiments. However, the teachings herein can be applied in a multitudeof different ways. In this description, reference is made to thedrawings wherein like parts are designated with like numeralsthroughout. The embodiments may be implemented in any device that isconfigured to display an image, whether in motion (e.g., video) orstationary (e.g., still image), and whether textual or pictorial. Moreparticularly, it is contemplated that the embodiments may be implementedin or associated with a variety of electronic devices such as, but notlimited to, mobile telephones, wireless devices, personal dataassistants (PDAs), hand-held or portable computers, GPSreceivers/navigators, cameras, MP3 players, camcorders, game consoles,wrist watches, clocks, calculators, television monitors, flat paneldisplays, computer monitors, auto displays (e.g., odometer display,etc.), cockpit controls and/or displays, display of camera views (e.g.,display of a rear view camera in a vehicle), electronic photographs,electronic billboards or signs, projectors, architectural structures,packaging, and aesthetic structures (e.g., display of images on a pieceof jewelry). MEMS devices of similar structure to those described hereincan also be used in non-display applications such as in electronicswitching devices.

Methods and devices are described herein related to analoginterferometric modulators. An analog interferometric modulator may bedriven to several different states each with different opticalproperties. Specifically methods and devices for calibrating andcontrolling the actuation of an analog interferometric modulator toachieve the various states are described.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, each pixel is in either a bright or dark state. In the bright(“relaxed” or “open”) state, the display element reflects a largeportion of the incident visible light to a user. When in the dark(“actuated” or “closed”) state, the display element reflects littleincident visible light to the user. Depending on the embodiment, thelight reflectance properties of the “on” and “off” states may bereversed. MEMS pixels can be configured to reflect predominantly atselected colors, allowing for a color display in addition to black andwhite.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a highly reflectivelayer positioned at a variable and controllable distance from anotherlayer which absorbs optical energy more or less uniformly across thevisible spectrum, forming a controllable optical gap. In one embodiment,the reflective layer may be moved between two positions. In the firstposition, referred to herein as the relaxed position, the movablereflective layer is positioned at a relatively large distance from theabsorbing layer. In the second position, referred to herein as theactuated position, the movable reflective layer is positioned moreclosely adjacent to the absorbing layer. Incident light that reflectsfrom the reflective layer interferes constructively or destructivelywithin the gap between the reflective and absorbing layers, whichdetermines whether the reflection from the pixel is in a highlyreflecting or a highly absorbing state.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several stratifiedlayers, which can include an electrode layer, such as indium tin oxide(ITO), an absorbing layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The absorbing layer can be formed from avariety of materials that are partially reflective such as variousmetals, semiconductors, and dielectrics. The absorbing layer can beformed of one or more layers of materials, and each of the layers can beformed of a single material or a combination of materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) to form columnsdeposited on top of posts 18 and an intervening sacrificial materialdeposited between the posts 18. When the sacrificial material is etchedaway, the movable reflective layers 14 a, 14 b are separated from theoptical stacks 16 a, 16 b by a defined gap 19. A highly conductive andreflective material such as aluminum may be used for the reflectivelayers 14, and these strips may form column electrodes in a displaydevice. Note that FIG. 1 may not be to scale. In some embodiments, thespacing between posts 18 may be on the order of 10-100 um, while the gap19 may be in the range 0-6000 Angstroms.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential (voltage) differenceis applied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by actuated pixel 12 b on the right in FIG. 1. Thebehavior is the same regardless of the polarity of the applied potentialdifference.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate interferometric modulators. Theelectronic device includes a processor 21 which may be any generalpurpose single- or multi-chip microprocessor such as an ARM®, Pentium®,8051, MIPS®, Power PC®, or ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. Note thatalthough FIG. 2 illustrates a 3×3 array of interferometric modulatorsfor the sake of clarity, the display array 30 may contain a very largenumber of interferometric modulators, and may have a different number ofinterferometric modulators in rows than in columns (e.g., 300 pixels perrow by 190 pixels per column).

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.For MEMS interferometric modulators, the row/column actuation protocolmay take advantage of a hysteresis property of these devices asillustrated in FIG. 3. An interferometric modulator may require, forexample, a 10 volt potential difference to cause a movable layer todeform from the relaxed state to the actuated state. However, when thevoltage is reduced from that value, the movable layer maintains itsstate as the voltage drops back below 10 volts. In the exemplaryembodiment of FIG. 3, the movable layer does not relax completely untilthe voltage drops below 2 volts. There is thus a range of voltage, about3 to 7 V in the example illustrated in FIG. 3, where there exists awindow of applied voltage within which the device is stable in eitherthe relaxed or actuated state. This is referred to herein as the“hysteresis window” or “stability window.” For a display array havingthe hysteresis characteristics of FIG. 3, the row/column actuationprotocol can be designed such that during row strobing, pixels in thestrobed row that are to be actuated are exposed to a voltage differenceof about 10 volts, and pixels that are to be relaxed are exposed to avoltage difference of close to zero volts. After the strobe, the pixelsare exposed to a steady state or bias voltage difference of about 5volts such that they remain in whatever state the row strobe put themin. After being written, each pixel sees a potential difference withinthe “stability window” of 3-7 volts in this example. This feature makesthe pixel design illustrated in FIG. 1 stable under the same appliedvoltage conditions in either an actuated or relaxed pre-existing state.Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

As described further below, in typical applications, a frame of an imagemay be created by sending a set of data signals (each having a certainvoltage level) across the set of column electrodes in accordance withthe desired set of actuated pixels in the first row. A row pulse is thenapplied to a first row electrode, actuating the pixels corresponding tothe set of data signals. The set of data signals is then changed tocorrespond to the desired set of actuated pixels in a second row. Apulse is then applied to the second row electrode, actuating theappropriate pixels in the second row in accordance with the datasignals. The first row of pixels are unaffected by the second row pulse,and remain in the state they were set to during the first row pulse.This may be repeated for the entire series of rows in a sequentialfashion to produce the frame. Generally, the frames are refreshed and/orupdated with new image data by continually repeating this process atsome desired number of frames per second. A wide variety of protocolsfor driving row and column electrodes of pixel arrays to produce imageframes may be used.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, voltages of opposite polarity than those described above can be used,e.g., actuating a pixel can involve setting the appropriate column to+V_(bias), and the appropriate row to −ΔV. In this embodiment, releasingthe pixel is accomplished by setting the appropriate column to−V_(bias), and the appropriate row to the same −ΔV, producing a zerovolt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows areinitially at 0 volts, and all the columns are at +5 volts. With theseapplied voltages, all pixels are stable in their existing actuated orrelaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. The same procedure can be employed for arrays ofdozens or hundreds of rows and columns. The timing, sequence, and levelsof voltages used to perform row and column actuation can be variedwidely within the general principles outlined above, and the aboveexample is exemplary only, and any actuation voltage method can be usedwith the systems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processes,including injection molding, and vacuum forming. In addition, thehousing 41 may be made from any of a variety of materials, including butnot limited to plastic, metal, glass, rubber, and ceramic, or acombination thereof. In one embodiment the housing 41 includes removableportions (not shown) that may be interchanged with other removableportions of different color, or containing different logos, pictures, orsymbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device. However, forpurposes of describing the present embodiment, the display 30 includesan interferometric modulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28, and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna for transmitting andreceiving signals. In one embodiment, the antenna transmits and receivesRF signals according to the IEEE 802.11 standard, including IEEE802.11(a), (b), or (g). In another embodiment, the antenna transmits andreceives RF signals according to the BLUETOOTH standard. In the case ofa cellular telephone, the antenna is designed to receive CDMA, GSM,AMPS, W-CDMA, or other known signals that are used to communicate withina wireless cell phone network. The transceiver 47 pre-processes thesignals received from the antenna 43 so that they may be received by andfurther manipulated by the processor 21. The transceiver 47 alsoprocesses signals received from the processor 21 so that they may betransmitted from the exemplary display device 40 via the antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. The above-described optimization may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 of each interferometric modulatoris square or rectangular in shape and attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is square or rectangular in shape and suspended from a deformablelayer 34, which may comprise a flexible metal. The deformable layer 34connects, directly or indirectly, to the substrate 20 around theperimeter of the deformable layer 34. These connections are hereinreferred to as support posts. The embodiment illustrated in FIG. 7D hassupport post plugs 42 upon which the deformable layer 34 rests. Themovable reflective layer 14 remains suspended over the gap, as in FIGS.7A-7C, but the deformable layer 34 does not form the support posts byfilling holes between the deformable layer 34 and the optical stack 16.Rather, the support posts are formed of a planarization material, whichis used to form support post plugs 42. The embodiment illustrated inFIG. 7E is based on the embodiment shown in FIG. 7D, but may also beadapted to work with any of the embodiments illustrated in FIGS. 7A-7Cas well as additional embodiments not shown. In the embodiment shown inFIG. 7E, an extra layer of metal or other conductive material has beenused to form a bus structure 44. This allows signal routing along theback of the interferometric modulators, eliminating a number ofelectrodes that may otherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. For example, such shielding allows the busstructure 44 in FIG. 7E, which provides the ability to separate theoptical properties of the modulator from the electromechanicalproperties of the modulator, such as addressing and the movements thatresult from that addressing. This separable modulator architectureallows the structural design and materials used for theelectromechanical aspects and the optical aspects of the modulator to beselected and to function independently of each other. Moreover, theembodiments shown in FIGS. 7C-7E have additional benefits deriving fromthe decoupling of the optical properties of the reflective layer 14 fromits mechanical properties, which are carried out by the deformable layer34. This allows the structural design and materials used for thereflective layer 14 to be optimized with respect to the opticalproperties, and the structural design and materials used for thedeformable layer 34 to be optimized with respect to desired mechanicalproperties.

The interferometric modulators described above are bi-stable displayelements having a relaxed state and an actuated state. The followingdescription, however, relates to analog interferometric modulators.These analog interferometric modulators have a range of states. Forexample, in one embodiment of an analog interferometric modulator, asingle interferometric modulator can have a red state, a green state, ablue state, a black state, and a white state. Accordingly, a singleinterferometric modulator is configured to have various states withdifferent light reflectance properties over a wide range of the opticalspectrum. Further, the optical stack of the analog interferometricmodulator may differ from the bi-stable display elements describedabove. These differences may produce different optical results. Forexample, in the bi-stable elements described above, the closed stategives the bi-stable element a black reflective state. The analoginterferometric modulator, however, has a white reflective state whenthe electrodes are in a similar position to the closed state of thebi-stable element.

FIG. 8 is an exemplary embodiment of an analog interferometric modulator800 having a three layer or electrode design. The modulator 800 includesan upper electrode 802. In one embodiment electrode 802 is a plate madeof metal. The upper electrode 802 may be stiffened using a stiffeninglayer 803. In one embodiment, the stiffening layer 803 is a dielectric.The stiffening layer 803 may be used to keep the upper electrode 802rigid and substantially flat. The modulator 800 also includes a middleelectrode 806 and a lower electrode 810. The three electrodes areelectrically insulated by insulating posts 804. The insulating posts 804also serve to hold middle electrode 806 between electrodes 802 and 810in a steady state when no electrostatic forces are present. The middleelectrode 806 has a stiffening layer 808 disposed thereon. In oneembodiment, the stiffening layer 808 is made of silicon oxynitride. Themiddle electrode 806 is configured to move in the area between upperelectrode 802 and lower electrode 810. The stiffening layer 808 keeps aportion of the middle electrode 806 rigid and substantially flat as itmoves between electrodes 802 and 810. In one embodiment, the stiffeninglayer 808 is disposed on the central portion of the middle electrode806. In this embodiment, the side portions of the middle electrode 806are able to bend as the middle electrode 806 moves. In FIG. 8, middleelectrode 806 is shown in an equilibrium position where the entireelectrode is substantially flat. As the middle electrode 806 moves awayfrom this equilibrium position, the side portions of the middleelectrode 806 will deform or bend. The side portions of the middleelectrode 806 implement an elastic spring force that applies a force tomove the middle electrode 806 back to the equilibrium position. Themiddle electrode 806 also serves as a metal mirror to reflect lightentering the structure through substrate 812. In an exemplaryembodiment, substrate 812 is made of glass. In one embodiment, the lowerelectrode 810 is an absorbing chromium layer. The lower electrode 810has a passivation layer 814 disposed thereon. In one embodiment, thepassivation layer 814 is a thin dielectric layer. In an exemplaryembodiment, the upper electrode 802 has a passivation layer disposedthereon. In one embodiment, the passivation layer is a thin dielectriclayer.

FIG. 9 is another exemplary embodiment of an analog interferometricmodulator 900 which is similar to modulator 800 of FIG. 8. The modulator900, however, has a larger stiffening layer 908 disposed on the middleelectrode 906 and further includes resistive elements 916 disposed onthe upper electrode 902. The upper electrode 902 has a stiffening layer903 disposed thereon. In one exemplary embodiment, the upper electrode902 is a metal and the stiffening layer 903 is a dielectric. Themodulator 900 also includes a lower electrode 910 with a thin dielectricpassivation 914 disposed thereon. The lower electrode 910 is disposed ona substrate 912. Resistive elements 916 provide a separator betweenupper electrode 902 and middle electrode 906. The insulating posts 904also serve to hold middle electrode 906 between electrodes 902 and 910in a steady state when no electrostatic forces are present. When middleelectrode 906 is moved toward upper electrode 902, resistive elements916 prevent the middle electrode 906 from coming into contact with theupper electrode 902. In one embodiment, middle electrode 906 includes aninsulating layer disposed on the bottom portion of the middle electrode906.

FIG. 10 is an exemplary embodiment of an analog interferometricmodulator 1100 with a control circuit 1120. The analog interferometricmodulator 1100 may be any one of modulator 800, modulator 900, or othersimilar design of analog interferometric modulator. Modulator 1100includes an upper electrode 1102, a middle electrode 1106, and a lowerelectrode 1110. The modulator 1100 further includes insulating posts1104 that insulate electrodes 1102, 1106, and 1110 from otherstructures.

The control circuit 1120 is configured to apply a voltage across theupper electrode 1102 and the lower electrode 1110. A charge pump 1118 isconfigured to selectively apply a charge to the middle electrode 1106.Using the control voltage 1120 and the charge pump 1118, electrostaticactuation of the middle electrode 1106 is achieved. The charge pump 1118is used to charge the middle electrode 1106 with a dose of electriccharge. The charged middle electrode 1106 then interacts with theelectric field created by control circuit 1120 between upper electrode1102 and the lower electrode 1110. The interaction of the charged middleelectrode 1106 and the electric field causes the middle electrode 1106to move between electrodes 1102 and 1110. The middle electrode 1106 canbe moved to various positions by varying the voltage applied by thecontrol circuit 1120. For example, a positive voltage V_(c) applied bycontrol circuit 1120 causes the lower electrode 1110 to be driven to apositive potential with respect to the upper electrode 1102, whichrepels the positively charged middle electrode 1106. Accordingly, apositive voltage V_(c) causes middle electrode 1106 to move toward upperelectrode 1102. Application of a negative voltage V_(c) by controlcircuit 1120 causes the lower electrode 1110 to be driven to a negativepotential with respect to the upper electrode 1102, which attractscharged middle electrode 1106. Accordingly a negative voltage V_(c)causes middle electrode 1106 to move toward lower electrode 1110. Themiddle electrode 1106 can accordingly move to a wide range of positionsbetween electrodes 1102 and 1110.

A switch 1122 is used to selectively connect or disconnect the middleelectrode 1106 from the charge pump 1118. It should be noted that othermethods known in the art besides a switch may be used to selectivelyconnect or disconnect the middle electrode 1106 from the charge pump1118 such as a thin film semiconductor, a fuse, an anti fuse, etc.

The switch 1122 may be configured to open and close to deliver aspecific charge to middle electrode 1106. A method of choosing a chargelevel is described below with respect to FIGS. 13 and 14. Further,switch 1122 may be configured to reapply a charge over time as thecharge leaks away or dissipates from the middle electrode 1106. In oneexemplary embodiment, charge is reapplied to middle electrode 1106according to a specified time interval.

FIG. 11 is an exemplary embodiment of an analog interferometricmodulator 1200 of the embodiment of FIG. 8. FIG. 11 illustrates a middleelectrode 1206 capable of being moved to various positions 1230-1236between upper electrode 1202 and lower electrode 1210. In oneembodiment, the middle electrode is moved according to the structuresand methods described with respect to FIG. 10.

The modulator 1200 is configured to selectively reflect certainwavelengths of light depending on the configuration of the modulator.The distance between the lower electrode 1210, which acts as anabsorbing layer, and the middle electrode 1206 changes theinterferometric properties of the modulator 1200. For example, themodulator 1200 is designed to be viewed on the substrate 1212 side ofthe modulator. Light enters the modulator 1200 through the substrate1212. Depending on the position of the middle electrode 1206, differentwavelengths of light are reflected back through the substrate 1212,which gives the appearance of different colors. For example, in position1230, the red wavelength of light is reflected, while the other colorsof light are absorbed. Accordingly, the interferometric modulator is ina red state. When the middle electrode moves to a position 1232, themodulator 1200 is in a green state and only the green wavelength oflight is reflected. When the middle electrode moves to a position 1234,the modulator 1200 is in a blue state and only the blue wavelength oflight is reflected. When the middle electrode moves to a position 1236,the modulator 1200 is in a white state and all the wavelengths of lightin the visible spectrum are reflected. It should be noted that one ofordinary skill in the art will recognize that the modulator 1200 cantake on different states and selectively reflect other wavelengths oflight or combinations of wavelengths of light depending on the materialsused in construction of the modulator and the position of the middleelectrode 1206.

FIG. 12 is an exemplary embodiment of an analog interferometricmodulator 1300 configured such that the middle electrode 1306 respondsin linear proportion to a voltage driven across upper electrode 1302 andlower electrode 1310. Accordingly, there is a linear relationshipbetween the voltage used to control the movement of the middle electrode1306 and the position of the middle electrode 1306 between electrodes1302 and 1310. In an ideal system, the electric field induced by thevoltage driven can be defined as follows:E=V/(δ₁+δ₂)  (1)where:

-   -   E is the electric field due to voltage V;    -   V is the voltage applied by control circuit 1320;    -   δ₁ is the effective distance between the lower electrode 1310        and the middle electrode 1306; and    -   δ₂ is the effective distance between the upper electrode 1302        and the middle electrode 1306.        Effective distance takes into account both the actual distance        (i.e., d₁ and d₂) between the two electrodes and the effect of        the passivation layers 1314 and 1303. The passivation layer        works to increase the effective distance and is calculated as        d_(∈)/∈ where d_(∈) is the thickness of the passivation layer        and c is the dielectric constant of the passivation layer.        Therefore, δ₁=d₁+d_(∈)/∈ and δ₂=d₂+d_(∈)/∈. It should be noted        that passivation layers 1303 and 1314 may have different        thicknesses and/or may be made of different materials.

The electric fields induced by the stored and isolated charge Q onmiddle electrode 1306 are given by:E ₂(x)=Q/(∈₀ *A)*(δ₁ +x)/(δ₁+δ₂)  (2)E ₁(x)=−Q/(∈₀ *A)*(δ₂ −x)/(δ₁+δ₂)  (3)where:

-   -   E₂=the electric field induced between upper electrode 1302 and        middle electrode 1306;    -   E₁=the electric field induced between lower electrode 1310 and        middle electrode 1306;    -   A=area of the electrodes;    -   ∈₀=the dielectric permittivity of a vacuum; and    -   x=the position of the middle electrode 1306 relative to the        equilibrium position of the middle electrode 1306 where no        voltage is applied by the control circuit 1320.

The force on the middle electrode due to the electric field is thengiven by:F _(E) =Q ²/(2∈₀ A)*(δ₁−δ₂+2x)/(δ₁+δ₂)+QV/(δ₁+δ₂)  (4)

Additionally, as described with respect to FIG. 8, the side portions ofthe middle electrode 806 may apply an elastic spring force F_(S) to themiddle electrode. This mechanical restoration force is given by:F _(S) =−Kx  (5)where:

-   -   K=the spring constant.    -   The force balance (F_(E) balancing F_(S)) is given by:

$\begin{matrix}{{{\frac{Q^{2}}{2\; ɛ_{0}A}\left( \frac{\delta_{1} - \delta_{2} + {2\; x}}{\delta_{1} + \delta_{2}} \right)} + \frac{QV}{\delta_{1} + \delta_{2}}} = {Kx}} & (6)\end{matrix}$

-   -   The position x of the middle electrode 1306 can then be solved        utilizing equation (6) as:

$\begin{matrix}{x = \frac{{\frac{Q^{2}}{2\; ɛ_{0}A}\left( \frac{\delta_{1} - \delta_{2}}{\delta_{1} + \delta_{2}} \right)} + \frac{QV}{\delta_{1} + \delta_{2}}}{K - \frac{Q^{2}}{\;{ɛ_{0}{A\left( {\delta_{1} + \delta_{2}} \right)}}}}} & (7)\end{matrix}$According to equation (7) the position of middle electrode 1306 in anideal system is linearly dependent on the voltage V applied.

It should also be noted that the sign of the denominator of equation (7)indicates whether the structure is stable or not (i.e., whether themiddle electrode 1306 will snap toward the closest outer electrode).Instability occurs if the electrostatic force on the middle electrode1306 induced by the stored charge itself overcomes the mechanicalrestoration force. The point of instability is a threshold charge Q_(c)given by the following equation:Q _(c)=√{square root over (K∈ ₀ A(δ₁+δ₂))}  (8)

If the middle electrode 1306 is not completely isolated electrically,the stored charge Q on the middle electrode 1306 may vary as itsposition between electrodes 1302 and 1310. This variation in Q canaffect the response of the middle electrode 1306 to a charge. Whenmiddle electrode 1306 is not completely isolated electrically, there areparasitic capacitances 1340, 1342 attached from it to each of the upperelectrode 1302 and the lower electrode 1310. Modulator 1300 isconfigured to account for the parasitic capacitances 1340, 1342 byincluding a capacitor 1344 connected in series with middle electrode1306 and in parallel with parasitic capacitances 1340, 1342. The effectof the capacitor 1344 in mitigating the parasitic capacitances 1340,1342 so as to allow the middle electrode 1306 to move in linear relationto the voltage applied by control circuit 1320 is described below. Thecapacitor 1344 clamps the total capacitance loading the middle electrode1306 and also blocks direct leakage paths for the charge on middleelectrode 1306.

C₂ is the capacitance of the gap between upper electrode 1302 and middleelectrode 1306 and C₁ is the capacitance of the gap between middleelectrode 1306 and lower electrode 1310.

$\begin{matrix}{C_{1} = \frac{ɛ_{0}A}{\delta_{1} + x}} & (9) \\{C_{2} = \frac{ɛ_{0}A}{\delta_{2} - x}} & (10)\end{matrix}$

The intended stored charge value Q₀ from application of a voltage V isgiven by the following equation:

$\begin{matrix}{Q_{0} = {\left( \frac{C_{S}V}{C_{1} + C_{2} + C_{s}} \right)\left( {C_{1} + C_{2}} \right)}} & (11)\end{matrix}$where:

-   -   C_(S)=the value of capacitor 1344.

After the charge Q₀ is placed on the middle electrode 1306, the effectof the applied bias voltage V across electrodes 1302 and 1310 on themiddle electrode 1306 can be calculated. The voltage dependent chargeQ_(MV) on middle electrode 1306 as a function of voltage and position ofthe middle electrode 1306 is given by the following equation:

$\begin{matrix}{Q_{MV} = \frac{{C_{S}\left( {{C_{P\; 1}C_{2}} - {C_{P\; 2}C_{1}}} \right)}V}{{\left( {C_{1} + C_{2}} \right)\left( {C_{S} + C_{P\; 1} + C_{P\; 2}} \right)} + {C_{S}\left( {C_{P\; 1} + C_{P\; 2}} \right)}}} & (12)\end{matrix}$where:

-   -   C_(P1)=the value of parasitic capacitance 1342; and    -   C_(P2)=the value of the parasitic capacitance 1340.        In the case where C_(P1)=C_(P2)=C_(P)>>C_(S), the voltage        dependent charge Q_(MV) simplifies to:

$\begin{matrix}{Q_{MV} = \frac{{C_{S}\left( {C_{2} - C_{1}} \right)}V}{2\left( {C_{1} + C_{2} + C_{S}} \right)}} & (13)\end{matrix}$The inequality holds where the value of C_(S) is chosen so as to lowerthe overall capacitance loading the middle electrode 1306. If, however,there is an imbalance in the parasitic capacitances 1340, 1342 (i.e.,C_(P1)=C_(P), C_(P2)=C_(P)+δC_(P)) the induced charge on middleelectrode 1306 is given by the equation:

$\begin{matrix}{Q_{MV} = \frac{{C_{S}\left( {C_{2} - C_{1} + {{\delta C}_{P}{C_{2}/C_{P}}}} \right)}V}{2\left( {C_{1} + C_{2} + C_{S}} \right)}} & (14)\end{matrix}$Compensating values, however, can be applied to make the parasiticcapacitances 1340 and 1342 approximately equal.

By combining the original stored charge Q₀ and the induced charge Q_(MV)the actual charge on middle electrode Q_(M) can be determined asQ_(M)=Q₀+Q_(MV). As long as capacitor 1344 is chosen such that equation(13) holds (i.e., C_(P1)=C_(P2)=C_(P)>>C_(S)), Q_(M) can be substitutedfor Q of equation (7) to give the position of the middle electrode 1306.Therefore, an appropriate capacitor 1344 will result in middle electrode1306 responding in approximately linear response to a voltage applied bycontrol circuit 1320. In one exemplary embodiment the capacitance ofcapacitor 1344 is approximately 1 fF. In another exemplary embodimentthe capacitance of capacitor 1344 is approximately 10 fF.

The design of modulator 1300 also limits the value of the associatedelectric field as the gap between the middle electrode 1306 and thelower electrode 1310 approaches 0. The limiting expression for theelectric field E_(lower) of an interferometric modulator utilizing 3electrodes is given by the following equation:E _(lower) =V/(δ₁+δ₂)−Q/(∈₀ A)  (15)which is the sum of the electric field due to the charge stored onmiddle electrode 1306 and the electric field applied by control circuit1320.

FIG. 13 illustrates the process 1400 by which the analog interferometricmodulator 1300 of FIG. 12 is calibrated. The calibration processconfigures the modulator 1300 so that the middle electrode 1306 willmove to a known position between upper electrode 1302 and lowerelectrode 1310 when a particular voltage is applied. Process 1400 isdescribed below with reference to FIG. 14 which illustrates aninterferometric modulator 1300 in various states of the calibrationprocess 1400.

When manufactured, the structure of any two given analog interferometricmodulators may have variations. There may be slight variations in someof the physical properties of two similar analog interferometricmodulators. For example, the spring constant K, the exact dimensions oflayers, and the spacing of layers may all differ. Variation can occurdue to process variations, temperature, and aging. In this case, toaccurately calculate the position x of middle electrode 1306 usingequation (7) would require knowledge of all of these variables. Further,if any variable differs between any given analog interferometricmodulators, the position x of middle electrode 1306 for each modulatormay differ when a known voltage is applied and a known charge is storedon middle electrode 1306. FIGS. 13 and 14 illustrate a process ofcalibrating an analog interferometric modulator such as one of theembodiment of FIG. 12 such that the approximate position of the middleelectrode 1306 at any given voltage applied by control circuit 1320 isknown without exact knowledge of the previously discussed variables.

At a step 1404, switch 1522 is closed to ground and a calibrationpotential V_(cal) is applied by control circuit 1520 as shown in FIG. 14a. At a next step 1408, a net negative charge is induced on the middleelectrode 1506. In one exemplary embodiment, a charge pump 1118 as shownwith respect to FIG. 10 induces the negative charge on the middleelectrode 1506 while switch 1522 is closed. As the charge is depositedon the middle electrode 1506, the charged electrode 1506 interacts withthe electric field created by the applied calibration potential andmoves toward the upper electrode 1502 as shown in FIG. 14 b. The forcefrom the electrostatic attraction is proportional to the square of theapplied electric field which increases as the distance between upperelectrode 1502 and middle electrode 1506 decreases, and as the charge onthe middle electrode 1506 increases. At a certain charge and distance,the force of the electric field overwhelms the elastic spring forceF_(S) on the middle electrode 1506. At this point, the middle electrode1506 “snaps” toward the upper electrode 1502. The middle electrode 1506then contacts resistive elements or posts 1516 as shown in FIG. 14 c.When contact is made, the potential difference between the upperelectrode 1502 and the middle electrode 1506 drops to the resistivedivide between the source resistance 1548 and the resistance of theposts 1516. The potential drops exponentially. As the potential drops,the mechanical restoring force or elastic spring force becomes greaterthan the electrostatic force on the middle electrode 1506. The middleelectrode 1506 then moves away from the resistive posts and thepotential difference between the upper electrode 1502 and the middleelectrode 1506 begins to climb back to the potential applied by thecontrol circuit 1120 between upper electrode 1502 and lower electrode1510. This results in an oscillation behavior. At a further step 1412 itis determined if the middle electrode 1506 is exhibiting oscillationbehavior. If it is determined the middle electrode 1506 is notexhibiting oscillation behavior, the process 1400 returns to step 1408and the charge continues to be induced on middle electrode 1506. When,at step 1412, it is determined that the middle electrode 1506 isexhibiting oscillation behavior, the process 1400 continues to a step1416. The oscillation behavior can be sensed using known methods in theart. At step 1416 the switch 1522 is opened to electrically isolatemiddle electrode 1506 and maintain an equilibrium charge Q_(e) on themiddle electrode 1506. When the switch 1522 is opened, the middleelectrode 1506 will remain at a small distance d_(g) from the resistiveposts 1516. In an exemplary embodiment the distance d_(g) is muchsmaller than the distance d₂ between upper electrode 1502 and middleelectrode 1506. Accordingly, when the middle electrode 1506 holds acharge Q_(e) and V_(cal) is applied by control circuit 1120, x≈d₂ andthe middle electrode 1506 will move a distance of approximately d₂toward upper electrode 1502. When no potential is applied by controlcircuit 1120, x=0 and the middle electrode will remain in an unmovedposition between upper electrode 1502 and bottom electrode 1510. Asindicated by equation (7) above, the movement of middle electrode 1506is linear in relation to the applied voltage, and therefore as one ofreasonable skill in the art will recognize, an equation for the positionx of the middle electrode 1506 can be determined using the two sets ofvalues for the position of middle electrode x in relation to an appliedvoltage (e.g., (0,0) and (d₂, V_(cal))).

In addition to sensing the oscillation, other variations for determiningwhen to disconnect switch 1522 at step 1412 to obtain an equilibriumcharge Q_(e) exist. In one embodiment of the calibration process, switch1522 is opened when the middle electrode 1506 makes contact with theresistive posts 1516. As the charge bleeds off the middle plate willseparate from the resistive posts 1516 and hold an equilibrium charge.In another embodiment, switch 1522 is opened when the middle electrode1506 acquires a sufficient charge to snap to resistive posts 1516, butbefore middle electrode 1506 makes contact with the resistive posts1516. The duration necessary to acquire sufficient charge may becalculated or established during a previous execution of the calibrationprocess.

It should be noted that charge on the middle electrode 1506 mayeventually bleed off and need to be re-charged periodically. The process1400 can be repeated in order to re-charge the middle electrode 1506. Inone embodiment, the re-charging process may also be modified such thatthe switch 1522 is only closed long enough to induce a charge greaterthan Q_(e) on the middle electrode 1506. The excess charge will thenbleed through the resistive posts 1516 to the appropriate Q_(e). In oneembodiment, the re-charging process may be scheduled for set intervals,such as after a set time interval. In another embodiment, the charge maybe monitored and the modulator recharged when the charge on the middleelectrode 1506 goes below a threshold value.

It should also be noted that in one embodiment, interferometricmodulator 1500 includes an additionally layer at the bottom of themiddle electrode 1506 which may prevent discharge of the middleelectrode 1506 if it contacts lower electrode 1510. In one embodiment,the additional layer is a thin insulator.

While the above process 1400 is described in the detailed description asincluding certain steps and are described in a particular order, itshould be recognized that these processes may include additional stepsor may omit some of the steps described. Further, each of the steps ofthe processes does not necessarily need to be performed in the order itis described.

While the above detailed description has shown, described and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the modulator or process illustrated may bemade by those skilled in the art without departing from the spirit ofthe invention. As will be recognized, the present invention may beembodied within a form that does not provide all of the features andbenefits set forth herein, as some features may be used or practicedseparately from others.

What is claimed is:
 1. A device for modulating light, comprising: afirst layer; a second layers, wherein a gap is present between the firstand second layer and the first layer and the second layer are stationaryrelative to each other; a third layer disposed in the gap between thefirst and second layer; a control circuit configured to selectivelyinduce an electric field between the first and second layers by applyinga bias voltage across the first layer and the second layer; and a chargepump configured to apply a fixed charge to the third layer; wherein thethird layer is electrically isolated after the fixed charge is applied,and wherein the control circuit is configured to cause the third layerto move within the gap by varying the bias voltage, wherein movement ofthe third layer is based on an electrostatic force between the electricfield and the fixed charge applied to the third layer and a mechanicalrestorative force acting on the third layer.
 2. The device of claim 1,wherein at least one resistive element is disposed on the bottom of thefirst layer.
 3. The device of claim 1, further comprising a switch forelectrically isolating the third layer.
 4. The device of claim 1,further comprising a thin film semiconductor or MEMS switch forelectrically isolating the third layer.
 5. The device of claim 1,wherein the third layer comprises an optical minor.
 6. The device ofclaim 1, wherein the second layer comprises a thin optical absorber. 7.The device of claim 1, wherein the control circuit is configured toapply a first voltage as the bias voltage when the device is in acalibration mode.
 8. The device of claim 7, wherein subsequent to thefirst voltage being applied, the electrostatic force is approximatelyequal to the mechanical restorative force.
 9. The device of claim 2,wherein the control circuit is configured to apply a first voltage asthe bias voltage when the device is in calibration mode, wherein thethird layer contacts the resistive element of the first layer subsequentto the first voltage being applied.
 10. The device of claim 1, whereinthe third layer is configured to move in linear proportion to the biasvoltage.
 11. A method of calibrating an analog interferometricmodulator, comprising: providing a first electrode and a secondelectrode, wherein a gap is present between the first electrode and thesecond electrode; applying a bias voltage across the first electrode andthe second electrode to induce an electric field between the firstelectrode and the second electrode; inducing a fixed charge on a thirdelectrode, wherein the third electrode is electrically isolated afterthe fixed charge is applied; and moving the third electrode within thegap by varying the bias voltage, wherein movement of the third electrodeis based on an electrostatic force between the electric field and thefixed charge applied to the third electrode and a mechanical restorativeforce acting on the third electrode.
 12. The method of claim 11, furthercomprising determining an amount of fixed charge comprising: providingat least one resistive element disposed on the first electrode; applyinga calibration voltage as the bias voltage to the first electrode andsecond electrode; and adjusting the fixed charge on the third electrodesuch that the electric field induced by the calibration voltage causesthe third electrode to contact the at least one resistive element of thefirst electrode.
 13. The method of claim 11, further comprisingadjusting the fixed charge on the third electrode such that theelectrostatic force about equals the mechanical restorative force. 14.The method of claim 11, wherein the inducing comprises inducing thefixed charge on the third electrode such that the displacement of thethird electrode responds in linear proportion to the bias voltage.
 15. Adevice for modulating light, comprising: first means for conductingcurrent; second means for conducting current, wherein a gap is presentbetween the first conducting means and the second conducting means;means for applying a bias voltage across the first conducting means andthe second conducting means to induce an electric field between thefirst conducting means and the second conducting means; and means forinducing a fixed charge on a third means for conducting current, whereinthe third means for conducting current is electrically isolated afterthe fixed charge is applied, and wherein the means for applying a biasvoltage is configured to cause the third means for conducting current tomove within the gap by varying the bias voltage, wherein movement of thethird means for conducting current is based on an electrostatic forcebetween the electric field and the fixed charge applied to the thirdmeans for conducting current and a mechanical restorative force actingon the third means for conducting current.
 16. The device of claim 15,wherein the applying means comprises a control circuit.
 17. The deviceof claim 15, wherein the inducing means comprises a charge pump.
 18. Thedevice of claim 15, further comprising a means for electricallyisolating the third conducting means.
 19. The device of claim 18,wherein the means for electrically isolating comprises a switch.
 20. Thedevice of claim 18, wherein the means for electrically isolatingcomprises a thin film semiconductor.
 21. The device of claim 15, whereinan insulating layer is disposed on the top of the second conductingmeans.
 22. The device of claim 15, wherein a stiffening layer isdisposed on the third conducting means.
 23. The device of claim 15,wherein a capacitor is in communication with the third conducting means.24. The device of claim 15, wherein the first conducting means comprisesan electrode, the second conducting means comprises an electrode, andthe third conducting means comprises an electrode.
 25. The device ofclaim 15, wherein means for inducing is further configured to induce thecharge on the third means for conducting current such that movement ofthe third conducting means is in linear proportion to the bias voltage.